1. Field of Use
This invention relates generally to rotary screw gas compressors used in refrigeration systems and to adjustably positionable slide valves used in such compressor to control their operation.
In particular, it relates to dual slide valves for a single rotary screw type gas compressor or the like and to improved control means for independently positioning the dual slide valves to thereby regulate both compressor capacity and compressor power input.
2. Description of the Prior Art
Rotary screw gas compressors used in refrigeration systems to compress refrigerant gas are available in two types, namely, those comprising two intermeshed helically-grooved main rotors or those comprising a single helically-grooved rotor, the grooves of which one engaged with one or more star-shaped or bladed gate rotors. In the latter type (called a "single screw" compressor) the main rotor is mounted for rotation in a bore in a compressor housing and is driven by an electric motor. The gate rotors are also mounted in the compressor housing and engage the main rotor. In such a single screw rotary compressor, each rotor groove, when engaged by a gate rotor blade, serves as a compression chamber in which uncompressed low-pressure gas received from a suction port in the housing is compressed and discharged as compressed high pressure gas to a discharge port in the housing. The gas pressure at the discharge port tends to vary substantially in response to variations in ambient temperatures resulting from seasonal or environmental temperature changes. If not corrected, the gas may be overcompressed in some situations and this results in extra work for the compressor and undesirable waste of electrical input power needed for operating the compressor. Accordingly, it is the practice to employ a slide valve which is movably positionable to adjust the location at which the discharge port opens; the preferred location being that at which internal gas pressure in the compression chambers on the rotor equals the condensing pressure in the refrigeration system in which the compressor is employed. Typically, the slide valve is mounted for axial movement in a recess adjacent and in communication with the rotor bore. The slide valve has a face which is complementary to and confronts the rotor surface in sliding sealed relationship. Means are employed to determine the most efficient position for the volume ratio slide valve and may take the form of means to sense these two pressure conditions, or to calculate positions, and to shift the slide valve axially in the proper direction for the proper distance until the equalization location is reached. Thus, if the discharge port on the slide is moved toward the discharge end of the rotor and compressor, the gas is trapped in the rotor grooves for a longer period of time and its volume is reduced as its pressure is increased i.e., the volume ratio is increased. On the other hand, if the discharge port on the slide valve is moved in the opposite direction, the volume ratio is lowered i.e., the internal cylinder pressure at the point of discharge is lowered, thereby causing the compressor volume ratio to decrease.
It is known to vary the capacity of dual or single rotor compressors by reducing or increasing the amount of gas trapped in each compression chamber by employing a slide valve to recirculate the gas back to the inlet. The following U.S. patents owned by the assignee of the present application illustrate the use of slide valves and controls therefor in dual rotor compressors to control the location at which gas is introduced to the compression chambers: 4,080,110; 4,005,949; 3,924,972; 3,869,227.
The hereinafter identified United Kingdom patents pertain to and disclose various features of a rotary screw gas compressor which employs a single helically grooved main rotor and star-shaped gate rotors engaged with the main rotor: 1,046,465; 1,288,603; 1,242,192; 1,345,946; 1,390,085; 1,388,537; 1,413,426; 1,407,135. A commercial embodiment of such a single screw rotary compressor is available from Hall-Thermotank Products Limited, Hythe Street, Dartford, Kent DA 1 1BU, England and is disclosed in that company's brochure entitled "The Hall Screw". The aforesaid commercial embodiment has slide valve members associated with the main rotor which are movable to regulate compressor capacity, but the built-in volume ratio needs to be maintained at full load and as capacity is reduced in order to achieve the most efficient operation.
U.S. Pat. No. 4,388,040 discloses a dual rotor compressor wherein a single slide valve and control means therefor operates to by-pass the suction port to control compressor capacity and the same slide valve has an extreme position wherein it is at compressor maximum load position wherein the discharge port is slightly enlarged.
U.S. Pat. Nos. 3,088,658 and 3,088,659 disclose a dual rotor compressor having two independently adjustable slide valves located on opposite sides of the dual rotors to regulate either the inbuilt pressure ratio or the capacity or both.
U.S. Pat. No. 3,869,227 owned by the assignee of the present application discloses a rotary screw type gas compressor employing two intermeshed helical main rotors, a single slide valve member associated with the two main rotors and movable to adjust the size of the opening of the high pressure gas discharge port to thereby regulate compressor capacity, and piston-cylinder type pneumatic activators to adjustably position the slide valve member.